1. Field of Invention
The present invention relates generally to improved brush seal designs for use in turbines such as steam and gas turbines and other apparatus that extracts work from the expansion of a working fluid.
2. Brief Description of the Prior Art
The use of axial flow elastic fluid turbines, such as axial flow steam turbines, plays a very important role in the production of electric power in our society. Often, in a typical power plant, there will be a number of steam turbines each driving one or more electrical power generators.
In general, each steam turbine comprises a rotatable shaft supported by bearings which are encased in a housing or casing. In order to rotate the rotor shaft using the momentum of superheated vapor (superheated steam for a steam turbine and hydrocarbon combustion gases for a “gas” turbine), a series of turbine stages are sequentially arranged along the axis of the shaft. A boiler, typically located external to the turbine casing, is provided for the purpose of generating steam for powering the turbine. External to the turbine casing are steam pipes which are used to conduct the steam from the boiler to the turbine. Turbines are typically classified by the pressure or pressure range at which the operate. Attached to the end of the turbine casing is a narrowing casing called a shroud. At the end of the shroud is a pipe (the snout pipe) that joins to the pipe from the boiler; sealing of the snout pipe to the steam supply pipe is effected by sealing rings called snout rings; the snout rings comprise two sets of rings that intermesh, one set disposed on the outside of the snout pipe and the other on the inside of the steam pipe into which the snout pipe leads. An example of this type of expansion joint is shown by Miller in U.S. Pat. No. 2,863,632 (the disclosure of which is incorporated herein by reference).
Each pressure stage of the turbine has a turbine rotor. Each rotor has a plurality of blades which radially extend a predetermined distance from the shaft, towards a circumferentially extending shroud band (i.e., cover) that is secured to the tenon portions of the blades. A stationary diaphragm is installed behind each rotor in a circumferential joint formed in the turbine casing. The inner structure of the diaphragm defines a ring of steam nozzles disposed circumferentially around the rotor. These nozzles are located at the same radial position as the blades in its associated rotor. The nozzles channel the steam (or other working fluid) entering that stage and channel it to the blades. To establish a “tip seal” with the shroud band of each turbine rotor, a ring of spillstrips segments are supported from the stationary diaphragm in each stage with packing that extends to the rotating shroud band (the band being attached to the blades, which are fixed to the rotor, which is rotating). As the steam travels through the turbine, a portion of its linear momentum is transformed into the angular momentum of the rotor blades at each turbine stage, thereby imparting torque to the turbine shaft. At downstream stages it is often necessary to increase the length of the rotor blades and the size of the associated diaphragms in order to extract kinetic energy from working fluid at a reduced pressure.
A major problem in turbine design relates to the quality of steam seals between the various stationary and rotating components along the steam flow path in the turbine. In general, there are several locations within the turbine where such seals must be established to ensure high turbine efficiency.
A first location where steam seals are required is between the outer portion of each rotor and its associated diaphragm have been effected using a segmented spillstrip ring of the type disclosed in U.S. Pat. No. 5,547,340, incorporated herein by reference. During start-up operations, when the rotor exhibits low frequency modes of operation about its axis, the tips of the rigid fin-like structure (e.g., fin seals) projecting along the spillstrip segments tend to rub against and/or cut into the shroud-band of the associated rotor, causing damage thereto during the start-up process. The only safeguards offered against such rubbing action has been to design the spillstrips so sufficient clearance exists between the tip portions of the fins on the spilistrips and the shroud band of the rotor. This approach, however, results in degradation of the tip seal, allowing steam to pass through the clearance area and not through and over the blades of the rotor, thereby reducing the performance of the turbine.
A second location where steam seals are required is between the casing and the turbine shaft. Creating seals over such regions has been addressed generally over the years by installing a segmented packing between the casing and the turbine shaft at each turbine stage. The packing typically consists of a first ring structure with multiple rows of fins (i.e., seal teeth) on one of the parts and a second structure with multiple rows of surface projections that correspond to the fins. The first ring structure typically is mounted from the associated diaphragm and the second ring structure typically is mounted to the turbine shaft. Together, the corresponding and registered rows of fins and projection structures create a labyrinth-type seal which presents a high impedance flow path to pressurized steam. However, during start up operation, low frequency modes of operation about the turbine axis tends to cause the tip portions of each row of fins to move radially outwardly and inwardly; in addition, differential thermal expansion caused as the hot working fluid is admitted to the stages and each heats up to operating temperature can exacerbate damage to the packing. To avoid rubbing and damage to such packing ring structures, it is necessary to design the fins and surface projections with sufficient clearance to avoid tip rubbing during start-up operation. This, however, necessarily degrades the quality of the labyrinth seal.
In U.S. Pat. Nos. 4,436,311 and 5,395,124 to Brandon (the disclosures of which are incorporated herein by reference), the problem of fin tip rubbing in packing ring design has been addressed by providing a retractable segmented packing ring structure between each rotor and turbine shaft. The manner in which the quality of the labyrinth seal is improved with this design is described as follows. During startup operation, when low frequency rotor vibration is predominant, the diaphragm-mounted packing ring segments are spring-biased in a radial direction away from the turbine shaft, reducing the risk of fin-tip portion rubbing and packing ring damage. As the rotor increases its angular speed, low frequency vibration is naturally reduced. The ring segments of the packing are forced to move closer (radially inward) to the turbine shaft by steam pressure, improving the quality of the labyrinth seal between the fins and the opposing corresponding surface projections, thereby improving the efficiency of the turbine.
An alternative solution to the problem of fin tip rubbing in packing is disclosed in UK Patent Application Publication No. GB 2 301 635 A. In this UK Patent Publication, a brush-type element is installed between a pair of fins extending from the packing ring segments mounted on the diaphragm. The function of the brush seal is to improve the quality of the labyrinth seal during all phases of operation. A major shortcoming with this design, however, is that during startup operations it does not provide a way of protecting the tips portions of the fin seals without designing a high degree of clearance into the design. Consequently, by virtue of such increased clearance requirements, the quality of the labyrinth seal provided by this prior art packing seal design is necessarily compromised.
Various other patents describe the use of brush seals in turbines, such as Ferguson in U.S. Pat. No. 4,971,336, Sanders et al. in U.S. Pat. No. 5,599,026, Bagepalli et al. in U.S. Pat. No. 5,474,306, and Skinner et al. in U.S. Pat. No. 5,749,584 (the disclosures of which are incorporated herein by reference). In these designs, the brush seals are designed be fixed and immovable. Many of the more recent brush seal designs provide the brushes canted at an angle from the radius of the turbine (the center being defined by the rotating turbine shaft). As Skinner et al. teach, existing machines with retractable seals (e.g., as described by Brandon) cannot be retrofit with a brush seal having canted bristles substituted for one of the labyrinth teeth while maintaining a 360° array brush seal. Skinner et al. teaches that the ends of each brush seal must be cut along the radius to provide each end of the segment with a flush surface for proper mating when the seals engage each other to form the ring structure, and due to the cant of the brushes relative to the flat end of the brush seal, the Skinner et al. design requires a gap in the brush seal.